Bacteriochlorophyll-protein, light-harvesting

Photosynthetic prokaryotes do not have chloroplasts. Their photosynthetic pigments are embedded in their cell walls. Some use bacteriochlorophyll for light harvesting. In the proteobacteria and archaea, light harvesting is accomplished by the protein rhodopsin, which acts as a photo-driven proton pump that fuels phosphorylation of ADP. 

The CD features of bacteriochlorophyll a in light-harvesting bacteriochlorophy 11-protein complexes from native bacteria is more interesting and is not compatible with Boxer s experiments. Bacteriochlorophyll(Bchl)-protein complexes exhibit various CD spectral profiles, depending on the species of bacteria and their culture conditions 201 206). Thus, CD of Bchl arises from the asymmetric environment in which the Bchl is situated, and from the specialized arrangement which affords a specific interaction between Bchl molecules, as well as from the asymmetry of Bchl... 

Figure 23-28 (A) Model of a light-harvesting chlorosome from green photosynthetic sulfur bacteria such as Chlorobium tepidum and species of Prosthecochloris. The chlorosome is attached to the cytoplasmic membrane via a baseplate, which contains the additional antenna bacteriochlorophylls (795 BChl a) and is adjacent to the trimeric BChl protein shown in (B) and near the reaction center. After Li et al.302 and Remigy et a/.304 (B) Alpha carbon diagram of the polypeptide backbone and seven bound BChl a molecules in one subunit of the trimeric protein from the green photosynthetic bacterium Prosthecochloris. For clarity, the magnesium atoms, the chlorophyll ring substituents, and the phytyl chains, except for the first bond, are omitted. The direction of view is from the three-fold axis, which is horizontal, toward the exterior of the molecule. From Fenna and Matthews.305 See also Li et al.302...

The basic structure of aU the component proteins making up the Light-harvesting (LH) complexes of purple photosynthetic bacteria is similar." " It consists of two polypeptides a- and jS-subunits) that fold into transmembrane Q -hehces and form a heterodimer that noncovalently binds bacteriochlorophyll... 

K Sauer and LA Austin (1978) Bacteriochlorophyll-protein complexes from the light-harvesting antenna of the photosynthetic bacteria. Biochemistry 17 2011-2019... 

The third example is the light-harvesting complex B800-850 obtained from the wild-type Rb. sphaeroides 2.4.1. This complex contains three BChls and one carotenoid (spheroidene) per protein subunit. The fluorescence excitation was obtained by monitoring the emission at 850 nm. Fig. 3 (C) shows the absorption and fluorescence excitation spectra in the 400-620 nm region, with the two spectra normalized at 590 nm (marked with ). The excellent match ofthe absorption and excitation spectra indicates that photoexcited spheroidene transfers energy to bacteriochlorophyll with a high efficiency. 

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

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